In the production of oil from subterranean oil reservoirs by various flooding techniques, especially waterflooding, it has become a common expedient to add various polymeric thickening agents to the water in order to increase its viscosity to a point where it approaches that of the crude oil which is to be displaced from the reservoir, so as to improve the displacement of the oil by the flooding liquid. The use of polymers for this purpose is often stated to be for mobility control.
Another problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability or "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to plug the high permeability zones wholly or partly and so divert the displacing fluid into the low permeability zones. Mechanical isolation of the high permeability zones has been attempted but vertical communication between reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success but here, the most serious drawback is the possibility of permanently closing still productive horizons.
From these early experiences, the desirability of designing a viscous slug capable of sealing off the more permeable layers so that the flooding liquid would be diverted to the underswept, tighter regions of the reservoir, became evident. This led to the use of various oil/water emulsions, gels and polymers for controlling the permeability of the formations in processes frequently referred to as "profile control", a reference to control of the vertical permeability profile of the reservoir.
Among the polymers so far proposed for improving water flood conformance are the polyacrylamides, polysaccarides, celluloses, acrylic resins, silicates and polyisocaynurates. One group of polymeric thickeners which has received considerable attention for use in flooding processes is the polysaccharides, particularly those produced by the action of bacteria of the genous Xanthomonas on carbohydrates. For example, U.S. Pat. No. 3,383,307 discloses the possibility of using gelled polysaocharides for mobility control purposes, U.S. Pat. Nos. 4,078,607 and 4,104,193 describe a method for improving the efficiency of waterflooding operations by a particular prehydration technique for polysaccharides and U.S. Pat. No. 4,413,680 describes the use of crosslinked polysaccharides for selective permeability control in oil reservoirs. The polysaccharides have been used in both their normal, uncrosslinked form as well as in the form of crosslinked metal complexes, as described, for example, in U.S. Pat. Nos. 3,757,863 and 3,810,882. U.S. Pat. No. 3,908,760 discloses the possibility of using reducible, metal containing complex anions as a source of the crosslinking agent.
Proposals have also been made for crosslinking other polymers including polyacrylamides and cellulose derivatives for use in controlling the permeability of oil-bearing formations, and crosslinking processes of this kind are described, for example, in U.S. Pat. Nos. 4,009,755, 4,069,869 and 4,413,680.
The one disadvantage of a number of existing crosslinking agents is that they are effective only certain, defined conditions of pH and these conditions may not be the same as those at which the polymer functions most effectively. For example, certain polysaccharides function most effectively under alkaline conditions, e.g. pH 9, while the crosslinking agents which form the strongest complexes with these saccharides, such as chromium and aluminum salts, form precipitates in strongly alkaline solutions and these precipitates are generally ineffective for crosslinking. In addition, highly charged metal cations such as Cr.sup.3+ have a high charge density by reason of their small ionic radius and this tends to distort the polymer with the result that the water may be squeezed out of the gel structure so that the gel is less stable than would be desirable. A further disadvantage of certain metal crosslinking agents is that they are relatively toxic and it would be desirable to avoid their introduction into the environment, particularly if there is a risk that the metal will enter aquifers from the oil-bearing formations. Finally, metal crosslinking agents such as chromium are relatively costly and it would be desirable to find cheaper alternatives. There is therefore a continuing need for the development of alternatives to existing crosslinking agents.